Solar collectors are typically "black" metal surfaces that absorb sunlight and are covered by glass to minimize heat losses. Sunlight is transmitted through the glass and is absorbed by the "black" surface. The "black" surface absorbs the shorter wavelength and emits longer wavelength energy which is absorbed by the glass thereby creating a greenhouse effect which increases the overall efficiency of the collector. A heat transfer fluid is circulated through the collector to collect the absorbed heat and transfer it to a remote location where it is utilized for space heating or the like.
The "black" surface is actually a selective coating which selectively absorbs the visible radiation and emits only a small portion of the generated infrared radiation. Although some of such coatings are truly black in appearance, many are black only in the sense of their ability to absorb visible light while emitting only small amounts of infrared radiation and may have the physical appearance of being transparent. The effectiveness of a selective coating is measured by the Figure of Merit which is the ratio of its absorptivity to its emissivity.
Numerous selective coatings have been developed. For example, selective paints have been developed. Although such painted surfaces work reasonably well, they often are not durable when exposed to the elements and to the high temperatures generated by the absorption process. Electrolytically produced oxide coatings on stainless steels have high Figures of Merit and overcome the drawbacks of painted surfaces but are presently very expensive.
Stainless steel, which are often used as solar collector plates, have been chemically blackened by immersion in molten dichromate salt baths. Chemical methods of blackening stainless steels are disclosed in U.S. Pat. Nos. 2,394,899; 2,499,764; 2,513,302; 2,537,035; 2,618,578; 3,437,532; 3,556,871; 3,900,601; and 3,963,530. The object of blackening in all but one of these patents is to provide a visually black surface for decorative or architectural purposes. U.S Pat. No. 3,900,601 discloses a wet process for treating metallized glass to provide a coating which rejects solar energy and is highly efficient in reflecting infrared radiation. None of these patent teaches that stainless steels and nickel-flash coated surfaces can be chemically treated to produce oxide coatings that are selective in the absorption of solar energy.
It has now been found that collector surfaces made of stainless steel or flashed coated with metallic nickel can be chemically oxidized under carefully controlled conditions to provide an oxide coating which selectively absorbs visible light and emits only small amounts of infrared radiation.